The present invention relates to a UHF mixing circuit, and more particularly to improvements in a UHF mixing circuit which has a diode mixer for mixing an input RF signal with an Oscillator signal and a double-tuned filter circuit for extracting either the sum or difference frequency resulting from mixing in the diode mixer.
The conventional UHF mixing circuit as shown in FIG. 1, comprises a diode mixer section and a double-tuned filter circuit connected thereto; the former being constructed of a coupling line 1 for electromagnetically coupling an RF signal into the mixer section, a mixer diode 2, a coupling line 3 for electromagnetically coupling an OSC signal into the mixer section, a capacitor 4, the capacitor being constructed of coils 5 through 7, and capacitors 8 and 9 capacitor. The RF and OSC signals flow into the mutually inductive coupling lines 1 and 3 of the loop circuit, and mixer diode 2 has the feature that it outputs non-linearly in response to its input. Because of this feature, the loop circuit generates an IF signal with a frequency that is the difference of the RF and OSC signal frequencies. This difference frequency signal is applied to the double-tuned filter circuit of coils 5-7 and capacitors 8-9. The IF signal which is the output of the double-tuned filter circuit is supplied to a VHF tuner, not shown. Since, in the prior-art example shown in FIG. 1, the double-tuned filter circuit is directly connected to the diode mixer, the impedance of the mixer diode 2 changes when subjected to input signals of different magnitudes, frequencies, etc. This has led to the disadvantage that an unfavorable state occurs in which the output waveform of the double-tuned filter circuit is non-linear to variations in the impedance of the mixer diode.